1. Field of the Invention
The present invention relates to temperature compensating circuits for compensating measured signals by analyzers, such as a gas analyzer, for example, for temperature variation.
2. Description of the Prior Art
Whenever accurate measurements are required, temperature compensation circuits are utilized to ensure that the accuracy of the measurement is not affected by changes in temperature in the environment of the measuring instrument.
A dissolved oxygen meter, for example, may have a plurality of displays, such as the display of dissolved oxygen in partial pressure ratio, and the display of dissolved oxygen (DO) in concentration, in addition to the display of the detected temperature itself. Each of the measured signals on which this multiple display is based must be individually temperature compensated. Other gas analyzers can analyze concentrations of a plurality of different kinds of gases. Here again, it is necessary to temperature compensate the detected signals of the concentrations of the respective gases.
Prior art temperature compensating circuits generally utilize a temperature sensitive (resistance) element r.sub.T in a feedback circuit of an operational amplifier "a" as shown in FIGS. 5A and 5B. The measured signal V to be compensated is supplied to the input circuit of said operational amplifier "a." Any one of many such input circuits may be selected, depending upon the characteristics of the measured signal. The gain of operational amplifier "a" depends upon and is changed with the temperature. The output signal V.sub.o is the input signal V subjected to temperature compensation.
Temperature compensating circuits having the above-described conventional construction have various kinds of disadvantages. One temperature compensating circuit, having an operational amplifier and a temperature sensitive (resistance) element r.sub.T is necessary for each measured signal V. In the case where temperature compensation is needed for a plurality of measured signals, the number of parts is remarkably increased, increasing not only the cost of production, but also complicating the construction and increasing the physical size.
Since complicated calculations are necessary for determining the characteristics of the temperature sensitive (resistance) element r.sub.T and the resistance values of the other resistors r.sub.a, r.sub.b, r.sub.c, depending upon the temperature characteristics of the measured signal V, the design and construction of a multiple sensing temperature compensating circuit according to this prior art teaching becomes quite complicated.
The circuit construction for each temperature compensating circuit may be quite different in structure, as shown in FIGS. 5A and B. Each must be selectively adopted, depending upon the polarity of the temperature characteristics of the measured signal V. In the case where temperature compensation for a plurality of measured signals different in polarity and temperature characteristics is required, the design and construction thereof is complicated even more.
Since the operational amplifier "a," an indispensable constituent element of the circuit, is apt to produce errors itself (temperature drift and the like) due to the influence of the surrounding temperature, these temperature compensation circuits still provide inaccuracy.
Applicants have recently developed a temperature compensating circuit capable of temperature compensation for a plurality of measured signals, by means of only one temperature sensitive element. This temperature compensating circuit has been described in Japanese Patent Application No. Sho 57-44869 (Japanese Patent Laid-Open No. Sho 58-160859), and Japanese Utility Model Application No. Sho 60-146072 (Japanese Utility Model Laid-Open No. Sho 62-53356).
However, even this temperature compensating circuit still has some shortcomings. Even though a temperature compensating circuit for carrying out temperature compensation for a plurality of measured signals is constructed, thereby requiring only one temperature sensitive (resistance) element, a comparatively expensive operational amplifier is still necessary. In addition, the complicated calculations for determining the characteristics of the temperature sensitive element r.sub.t and the resistance values for r.sub.a, r.sub.b and r.sub.c are still necessary. The temperature compensating circuit must still be adapted to the temperature characteristics of the measured signal V.
The present invention is an improvement over the above-described prior work. It is an object of the present invention to develop and provide a temperature compensating circuit capable of remarkably reducing a number of constituent parts, simplifying and miniaturizing inexpensively the construction, remarkably simplifying the design procedure corresponding to the temperature characteristics of the measured signal, and carrying out temperature compensation remarkably more accurate than the prior art.